Gokhan Danabasoglu, Senior Scientist
CCSM Distinguished Achievement Award
Gokhan Danabasoglu is the recipient of the 2009 CCSM Distinguished Achievement Award. Gokhan has been instrumental in developing every version of the ocean component, starting from CCSM1. With Gokhan as co-chair of the Ocean Working Group, the ocean model component for CCSM4 was delivered on time, with everything incorporated as planned and more. It is our opinion that no one has been more key to this, and past versions of the ocean model than Gokhan. [more]
Danabasoglu, G., W. G. Large, and B. P. Briegleb, 2010: Climate impacts of parameterized Nordic Sea overflows. JGR - Oceans (submitted). [Download (13.3Mb)]
Briegleb, B. P., G. Danabasoglu, and W. G. Large, 2010: An overflow parameterization for the ocean component of the Community Climate System Model. NCAR Technical Note. [Download (1.6Mb)]
Danabasoglu, G., S. Peacock, K. Lindsay, and D. Tsumune, 2009: Sensitivity of CFC-11 uptake to physical initial conditions and interannually varying surface forcing in a global ocean model. Ocean Modelling (submitted). [Download (11.6Mb)]
Sensitivity of the oceanic chlorofluorocarbon CFC-11 uptake to physical initial conditions and surface dynamical forcing (heat and salt fluxes and wind stress) is investigated in a global ocean model used in climate studies. Two different initial conditions are used: a solution following a short integration starting with observed temperature and salinity and zero velocities, and the quasi-equilibrium solution of an independent integration. For surface dynamical forcing, recently developed normal-year and interannually varying (1958-2000) data sets are used. The model CFC-11 global and basin inventories, particularly in the normal-year forcing case, are below the observed mean estimates, but they remain within the observational error bars. Column inventory spatial distributions indicate nontrivial differences due to both initial condition and forcing changes, particularly in the northern North Atlantic and Southern Ocean. These differences are larger between forcing sensitivity experiments than between the initial condition cases. The comparisons along the A16N and SR3 WOCE sections also show differences between cases. However, comparisons with observations do not clearly favor a particular case, and model - observation differences remain much larger than model - model differences for all simulations. The choice of initial condition does not significantly change the CFC-11 distributions. Both because of locally large differences between normal-year and interannually varying simulations and because the dynamical and CFC-11 forcing calendars are synchronized, we favor using the more realistic interannually varying forcing in future simulations, given the availability of the forcing data sets.
Danabasoglu, G., and Peter R. Gent, 2009: Equilibrium climate sensitivity: Is it accurate to use a slab ocean model? J. Climate (in press). [Download (0.6Mb)]
The equilibrium climate sensitivity of a climate model is defined as the globally-averaged surface temperature response to a doubling of carbon dioxide. This is virtually always calculated by using a slab model for the upper ocean. The question is whether this is an accurate assessment for the climate model as a whole, which includes a full depth ocean component. This question has been answered for the low resolution version of the Community Climate System Model version 3. The answer is that the equilibrium climate sensitivity using the full depth ocean model is 0.15C higher than using the slab ocean model, which is a rather small increase. Given that these sensitivity calculations have a standard deviation of 0.1C due to inter-annual variability, this implies that the standard practice of using a slab ocean model does give a good estimate of the true equilibrium climate sensitivity.
Danabasoglu, G., 2008: On multi-decadal variability of the Atlantic meridional overturning circulation in the Community Climate System Model version 3 (CCSM3). J. Climate (in press). [Download (5.2Mb)]
Multi-decadal variability of the Atlantic meridional overturning circulation (MOC) is investigated diagnostically in the NCAR CCSM3 present-day simulations, using the highest (T85x1) resolution version. This variability has a 21-year period and is present in many other ocean fields in the North Atlantic. In MOC, the oscillation amplitude is about 4.5 Sv, corresponding to 20% of the mean maximum MOC transport. The northward heat transport (NHT) variability has an amplitude of about 0.12 PW, representing 10% of the mean maximum NHT. In sea surface temperature (SST) and salinity (SSS), the peak-to-peak changes can be as large as 6-7C and 3 psu, respectively. The Labrador Sea region is identified as the deep water formation (DWF) site associated with the MOC oscillations. In contrast with some previous studies, temperature and salinity contributions to the total density in this DWF region are almost equal and in-phase. The heat and freshwater budget analyses performed for the DWF site indicate a complex relationship between the DWF, MOC, North Atlantic Oscillation (NAO), and subpolar gyre circulation anomalies. Their complicated interactions appear to be responsible for the maintenance of the multi-decadal oscillation. In these interactions, the atmospheric variability associated with the model's NAO plays a prominent role. In particular, the NAO modulates the subpolar gyre strength and appears to be responsible for the formation of the temperature and salinity anomalies that lead to positive / negative density anomalies at the DWF site. In addition, the wind stress curl anomalies occurring during the transition phase between the positive and negative NAO states produce fluctuations of the subtropical-subpolar gyre boundary, thus creating mid-latitude SST and SSS anomalies. Comparisons with observations show that neither the pattern nor the magnitude of the SST variability is realistic.
Jochum, M., G. Danabasoglu, M. Holland, Y.-O. Kwon, and W. G. Large, 2008: Ocean viscosity and climate. J. Geophys. Res., 113, C06017, doi:10.1029/2007JC004515. [Download (5.6Mb)]
The impacts of parameterized ocean viscosity on climate are explored using three 120 year integrations of a fully coupled climate model. Reducing viscosity leads to an improved ocean circulation at the expense of increased numerical noise. Five domains are discussed in detail: the equatorial Pacific, where the emergence of tropical instability waves improves the cold tongue bias; the Southern Ocean, where the Antarctic Circumpolar Current increases its kinetic energy but reduces its transport; the Arctic Ocean, where an improved representation of the Atlantic inflow leads to an improved sea-ice distribution; the North Pacific, where the more realistic path of the Kuroshio leads to improved tracer distribution across the mid-latitude Pacific; and the northern marginal seas, whose better represented boundary currents lead to an improved sea-ice distribution. Although the ocean circulation and sea-ice distribution improve, the oceanic heat uptake, the poleward heat transport, and the large scale atmospheric circulation are not changed significantly. In particular, the improvements to the equatorial cold tongue did not lead to an improved representation of tropical precipitation or El Nino.
Kleypas, J. A., G. Danabasoglu, and J. M. Lough, 2008: Potential role of the ocean thermostat in determining regional differences in coral reef bleaching events. Geophys. Res. Lett., 35, L03613, doi:10.1029/2007GL032257. [Download]
Several negative feedback mechanisms have been proposed by others to explain the stability of maximum sea surface temperature (SST) in the western Pacific warm pool (WPWP). If these ``ocean thermostat'' mechanisms effectively suppress warming in the future, then coral reefs in this region should be less exposed to conditions that favor coral reef bleaching. In this study we look for regional differences in reef exposure and sensitivity to increasing SSTs by comparing reported coral reef bleaching events with observed and modeled SSTs of the last fifty years. Coral reefs within or near the WPWP have had fewer reported bleaching events relative to reefs in other regions. Analysis of SST data indicate that the warmest parts of the WPWP have warmed less than elsewhere in the tropical oceans, which supports the existence of thermostat mechanisms that act to depress warming beyond certain temperature thresholds.
Danabasoglu, G., R. Ferrari, and J. C. McWilliams, 2008: Sensitivity of an ocean general circulation model to a parameterization of near-surface eddy fluxes. J. Climate, 21, 1192-1208. [Download (2.5Mb)]
A simplified version of the near-boundary eddy flux parameterization of Ferrari and McWilliams (2007) has been implemented in the NCAR Community Climate System Model (CCSM3) ocean component for the surface boundary only. This scheme includes the effects of diabatic mesoscale fluxes within the surface layer. The experiments with the new parameterization show significant improvements compared to a control integration that tapers the effects of the eddies as the surface is approached. Such surface tapering is typical of present implementations of eddy transport in some current ocean models. The comparison is also promising versus available observations and results from an eddy-resolving model. These improvements include the elimination of strong, near-surface, eddy-induced circulations and a better heat transport profile in the upper-ocean. The experiments with the new scheme also show reduced abyssal cooling and diminished trends in the potential temperature drifts. Furthermore the need for any ad-hoc, near-surface taper functions is eliminated. The impact of the new parameterization is mostly associated with the modified eddy-induced velocity treatment near the surface. The new parameterization acts in the depth range exposed to enhanced turbulent mixing at the ocean surface. This depth range includes the actively turbulent boundary layer and a transition layer underneath, composed of waters intermittently exposed to mixing. The mixed layer, i.e. the regions of weak stratification at the ocean surface, is found to be a good proxy for the sum of the boundary layer depth and transition layer thickness.
Wu, W., G. Danabasoglu, and W. G. Large, 2007: On the effects of parameterized Mediterranean overflow on North Atlantic ocean circulation and climate. Ocean Modelling, 19, 31-52, doi:10.1016/j.ocemod.2007.06.003.
A parameterized Mediterranean overflow, based on the marginal sea boundary condition of Price and Yang (1998), has been implemented in the ocean component of the Community Climate System Model to represent exchanges through the Strait of Gibraltar, associated entrainment and intrusion of overflow product water into the Atlantic. Previously, in coarse resolution model versions with a closed Strait, this physics has been either missing in uncoupled configurations or both only partially and unphysically treated as a surface salt exchange when fully coupled. Parameter choices are evaluated by comparing climatologically forced solutions to observations and process model results. The two major criteria satisfied by the implementation in a fully coupled climate model and a global ocean model are stable solutions and projection of the overflow signal across the Atlantic basin at about 1000 m depth. Both of these configurations are low resolution, and in both the transports of inflow, source and entrainment water are all within the range of observed estimates, but there is too little product water. This bias is attributed to inadequate modeling of water masses in the Mediterranean source region. Nevertheless, the properties of the product water differ little from observed estimates and both the uncoupled and coupled models develop a Mediterranean salt tongue that spreads west and south from the Strait with a signature reminiscent of the observed hydrography. The improvements relative to either blocking the Strait, or excavating a too wide channel are presented. In the coupled solution, the impact of the improved overflow physics on the global climate is minimal, with North Atlantic sea surface temperatures and heat fluxes changing generally by less than 1°C and 15 W m-2, respectively. However, there is interesting spatial variability in the coupling strength, which ranges between ±20 W m-2 °C-1 in the coupled case.
Doney, S. C., S. Yeager, G. Danabasoglu, W. G. Large, and J. C. McWilliams, 2007: Mechanisms governing interannual variability of upper ocean temperature in a global ocean hindcast simulation. J. Phys. Oceanogr., 37, 1918-1938.
We quantify the interannual variability in upper ocean (0-400m) temperature and governing mechanisms for the period 1968-1997 from a global ocean hindcast simulation driven by atmospheric reanalysis and satellite data products. The unconstrained simulation exhibits considerable skill in replicating the observed interannual variability in vertically integrated heat content, estimated from hydrographic data, and monthly satellite sea surface temperature and sea surface height data. Globally, the most significant interannual variability modes arise from El Nino-Southern Oscillation and the Indian Ocean zonal mode, with substantial extension beyond the tropics into the mid-latitudes. In the well-stratified tropics and subtropics, net annual heat storage variability is driven predominately by the convergence of the advective heat transport, mostly reflecting velocity anomalies times the mean temperature field. Vertical velocity variability is caused by remote wind forcing, and subsurface temperature anomalies are governed mostly by isopycnal displacements (heave). The dynamics at mid- to high-latitudes are qualitatively different and vary regionally. Interannual temperature variability is more coherent with depth because of deep winter mixing and variations in western boundary currents and the Antarctic Circumpolar Current that span the upper thermocline. Net annual heat storage variability is forced by a mixture of local air-sea heat fluxes and the convergence of the advective heat transport, the latter resulting from both velocity and temperature anomalies. And density compensated temperature changes on isopycnal surfaces (spice) are quantitatively significant.
Danabasoglu, G., and J. Marshall, 2007: Effects of vertical variations of thickness diffusivity in an ocean general circulation model. Ocean Modelling, 18, 122-141, doi:10.1016/j.ocemod.2007.03.006.
The effects of a prescribed surface intensification of the thickness (and isopycnal) diffusivity on the solutions of an ocean general circulation model are documented. The model is the coarse resolution version of the ocean component of the National Center for Atmospheric Research (NCAR) Community Climate System Model version 3 (CCSM3). Guided by the results of Ferreira, Marshall, and Heimbach (2005), we employ a vertical dependence of the diffusivity which varies with the stratification, N2, and is thus large in the upper ocean and small in the abyss. We experiment with vertical variations of diffusivity which are as large as 4000 m2 s-1 within the surface diabatic layer, diminishing to 400 m2 s-1 or so by a depth of 2 km. The new solutions compare more favorably with the available observations than those of the control which uses a constant value of 800 m2 s-1 for both thickness and isopycnal diffusivities. These include an improved representation of the vertical structure and transport of the eddy-induced velocity in the upper-ocean North Pacific, a reduced warm bias in the upper ocean, including the equatorial Pacific, and improved southward heat transport in the low- to mid-latitude Southern Hemisphere. There is also a modest enhancement of abyssal stratification in the Southern Ocean.